Get a 50A Female to 30A Male adapter (WalMart, CW, Amazon, etc.) Plug the power cable from your FW into the adapter, and the adapter into the 30A socket on one of your generators. The generator is taking the place of a 30A socket you'd find in a campground. If you try to draw more than 30A (2nd A/C, A/C plus microwave plus hair dryer, etc.) the breaker on the generator should trip. This will charge your FW battery, just as plugging into shore power would.
Your second generator becomes a spare. There's no simple way to parallel two generators to produce more power (i.e., 60A from two 30A generators), unless that capability is built into the generators.